Battery charger with automatic voltage detection

ABSTRACT

A battery charger is disclosed that is configured to be connected to an external battery by way of external battery cables. In accordance with an important aspect of the invention, the battery charger is configured with automatic voltage detection which automatically determines the nominal voltage of the battery connected to its battery charger terminals and charges the battery as a function of the detected nominal voltage irrespective of the nominal voltage selected by a user. Various safeguards are built into the battery charger to avoid overcharging a battery. For battery chargers with user selectable nominal battery voltage charging modes, battery charger is configured to over-ride a user selected battery voltage mode if it detects that the battery connected to the battery charger terminals is different than the user selected charging mode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a battery charger configured to beconnected to an external battery by way of external battery cables andmore particularly to a battery charger with automatic voltage detectionwhich automatically determines the nominal voltage of the batteryconnected to its battery charger terminals and charges the battery as afunction of the detected nominal voltage irrespective of the nominalvoltage selected by a user.

2. Description of the Prior Art

Various battery chargers for charging different types of batteries areknown in the art. Examples of such battery chargers are disclosed inU.S. Pat. Nos. 5,729,115; 6,384,575; 6,625,477; and 7,468,596. Such abattery charger is also disclosed in US Patent Application PublicationNo. US 2007/0247105 A1, all hereby incorporated by reference.

Since each different type of battery needs to be charged according to aspecific charging algorithm for the specific battery, it is necessaryfor the battery charger to be properly configured for the battery typeand nominal voltage of the battery connected to its battery chargerterminals. Some known battery chargers require the user to determine thebattery type and nominal voltage of a battery connected to its batterycharger terminals. Such battery chargers require the user to manuallyconfigure the battery charger. Other known battery chargersautomatically determine the battery type and the nominal voltage of thebattery connected to its battery charger terminals and automaticallyconfigure the battery charger.

Various techniques are known for battery chargers for automaticallydetermining the nominal voltage of the battery connected to the batterycharger. For example, U.S. Pat. Nos. 6,384,575; 6,384,575 and US PatentApplication Publication No. US 2007/0247105 A1 all disclose batterychargers which distinguish different types of batteries by size. Ingeneral, these battery chargers include multiple charging pockets. Thepockets are configured to receive different size batteries withdifferent nominal voltages. These chargers merely need to sense whichpocket has a battery connected to it in order to determine the batteryvoltage. However, such a technique is not applicable to battery chargersthat are configured to be connected to external batteries by way ofexternal battery cables.

U.S. Pat. No. 6,625,477 discloses a different technique for determiningthe nominal voltage of a battery connected to its battery chargerterminals. The battery charger disclosed in the '477 patent isconfigured to identify the nominal voltage of specially configuredbatteries which include an identification contact. The battery chargerincludes a plurality of tap voltages juxtaposed so that when the batteryis received in the battery charger, the identification contact on thebattery will be connected to a tap voltage depending on its size andthus nominal voltage. Again, such a technique is not applicable tobattery chargers that are configured to be connected to externalbatteries by way of external battery cables.

U.S. Pat. No. 5,729,115 discloses yet another technique for determiningthe nominal voltage of a battery connected to its battery chargerterminals. In this technique, the battery charger includes a sensingcontact in addition to the positive and negative battery terminals. Thesensing contact is juxtaposed adjacent to the positive battery chargerterminal. Whenever a battery is inserted into the battery charger, thesensing contact is configured so that it will be in contact with thepositive battery terminal for a first type of battery and will not be incontact with the positive battery terminal for a second type of battery.The battery charger senses the voltage at the sensing contact and makesa determination of the nominal voltage of the battery connected to itspositive and negative terminals based on the voltage at the sensingcontact. This technique, like the techniques discussed above, is notapplicable to battery chargers that are configured to be connected toexternal batteries by way of external battery cables.

Thus, there is a need for a battery charger that is configured to beconnected to an external battery by way of external cables that canautomatically determine the nominal voltage of the battery connected toits battery charger terminals.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to battery charger configured tobe connected to an external battery by way of external battery cables.In accordance with an important aspect of the invention, the batterycharger is configured with automatic voltage detection whichautomatically determines the nominal voltage of the battery connected toits battery charger terminals and charges the battery as a function ofthe detected nominal voltage irrespective of the nominal voltageselected by a user. Various safeguards are built into the batterycharger to avoid overcharging a battery. For battery chargers with userselectable nominal battery voltage charging modes, the battery chargeris configured to over-ride a user selected battery voltage mode if itdetects that the battery connected to the battery charger terminals isdifferent than the user selected charging mode.

DESCRIPTION OF THE DRAWING

These and other advantages of the present invention will be readilyunderstood with reference to the following specification and attacheddrawing wherein:

FIG. 1 is a block diagram of a battery charger that is configured to beconnected to an external battery by way of external battery cables anddetermine the nominal voltage of a battery connected to its batterycharger terminals.

FIGS. 2-7 are software flow diagrams for the battery charger illustratedin FIG. 1 for detecting the nominal voltage of the battery connected tothe battery charger terminals of the battery charger.

DETAILED DESCRIPTION

The present invention relates to a battery charger that is configured tobe connected to an external battery by way of external cables, such asbattery cables. In accordance with an important aspect of the invention,the battery charger is configured with automatic voltage detection whichautomatically determines the nominal voltage of the battery connected toits battery charger terminals and charges the battery as a function ofthe detected nominal voltage irrespective of the nominal voltageselected by a user. Various safeguards are built into the batterycharger to avoid overcharging a battery. For battery chargers with userselectable nominal battery voltage charging modes, the battery chargeris configured to over-ride a user selected battery voltage mode if itdetects that the battery connected to the battery charger terminals isdifferent than the user selected charging mode.

The present invention can be implemented on virtually any batterycharger, for example, the battery charger, illustrated in FIG. 1 andidentified with the reference numeral 10, disclosed in commonly ownedco-pending US Patent Application Publication No. 2005/0088144 A1, herebyincorporated by reference. The battery charger 10 includes amicroprocessor/microcontroller 12 and a pair of battery chargerterminals, generally identified with the reference numeral 17. Moreover,the principles of the present invention are applicable to any batterytypes, such as lead acid, absorbed glass mat (AGM), spiral wound AGMvalve regulated lead acid (VRLA), flooded cell and deep—cycle batteries,generally identified by the reference numeral 16.

A battery charger is described and illustrated that is configured todetect whether a battery with a nominal 6 volts or 12 volts is connectedto its terminals. However, the principles of the invention areapplicable to detecting the nominal voltages of virtually any batteryconnected to the battery charger terminals. For example, the principlesof the invention can be used to determine the nominal voltages of 8, 24,36, 48, 60 volt batteries as well as the nominal voltage of virtuallyany battery. More particularly, the battery charger in accordance withthe present invention is able to determine the nominal voltage of anexternal battery connected to its battery charger terminals by takingcertain voltage measurements under certain conditions. For batterieshaving nominal voltages other than 6 volts/12 volts, the voltage levelsset forth in FIGS. 2-7 are scalable. For example, for batteries withnominal voltages other than 6 volt/12 volt, the voltage levelsillustrated in FIGS. 2-7 may be scaled in accordance with the ratio ofthe nominal voltages to the 6 volt/12 volt voltage levels illustrated inFIGS. 2-7.

As used herein, the battery voltage measurement refers to the opencircuit battery voltage. In other words, the voltage across the batterycharger terminals with no current flowing from the battery chargerthrough the battery charger terminals. The principles of the presentinvention are also applicable to closed circuit battery voltagemeasurements in which the battery voltage measurements are made while anelectrical current is flowing from the battery charger and through thebattery terminals and thus includes the so called “IR” drop across thebattery cables and battery terminals.

Turning to FIG. 2, on power-up, the battery charger 10 initializes thesystem, as indicated by logic blocks 18 and 20. More specifically, theinput/output (I/O) ports on the microprocessor 12 are initialized alongwith the system clock. The analog to digital converters, which may beexternal or on board with the microprocessor 12, are calibrated and allsystem variables are initialized. In addition all LEDs are tested andset to their initial state. The variable “Initial Battery Check” isset=1 and the various timers as discussed below are initialized.

After initialization, the system proceeds to step 22 and checks thevoltage across its battery charger terminals 17. Specifically, thebattery charger terminals 17 are coupled to the ADC. The analog batteryvoltage is converted to a digital value and compared with apredetermined value. In other words, the system “reads” the voltageacross the battery charger terminals and determines whether the voltageacross the battery charger terminals is greater than a nominal amount,for example, 0.2 volts DC, a value simply indicative of whether abattery is connected across the battery charger terminals. If there isno battery connected across the battery charger terminals, the systemloops back to steps 22 and 24 and waits for a battery to be connected tothe battery charger terminals.

Once the system detects that a battery is connected across the batterycharger terminals 17, the system initially makes a simple voltagemeasurement in order to determine whether the battery connected to itsbattery charger terminals has a nominal 6 volts or a nominal 12 volts.More specifically, the system initially determines in step 24 whetherthe voltage across the battery charger terminals is greater than thenominal amount, illustrated in FIG. 2 as 0.2 volts DC. If so, the systemassumes a battery of unknown nominal voltage is connected across itsbattery charger terminals.

For battery chargers equipped with user selectable mode switches, thesystem determines the position of the mode switch in step 28. Such modeswitches are used to initially select the charging algorithm to bedelivered by the battery charger to the battery connected to its batterycharger terminals 17. As described herein, the mode switch (not shown)is user selectable between 6 volts and 12 volts.

In step 26, the system measures the battery voltage and compares themeasured voltage with the voltage designated by the position of the modeselector switch to determine if the user selectable mode switch is setfor the correct mode. If the mode switch is set at 12 volts, the systeminitially determines if the measured voltage is less than 17 volts DC or8.5 volts DC if the mode switch is set in a 6 volt mode. If the measuredvoltage is greater than 17 volts DC, the system checks in step 28whether the user selectable mode switch was set for the 6 Volt mode. Ifthe measured voltage is greater than 8.5 volts DC the user selectablemode switch set for the 6 volts DC mode, the system assumes that a 12volt DC battery is connected to the battery charger terminal 17 In thiscase, the system over-rides the user selected position for the modeswitch and configures the system to charge battery in accordance withthe 12 volt algorithm, as set forth below, as indicated in step 30 andproceeds to step 32. In addition, the system optionally toggles one ormore LEDs indicating the over-ride of the user selected mode position instep 35. The system then loops back to the logic block 22 and repeatssteps 24 and 26. This time, since the battery mode was automatically setfor the 12 volt mode by the battery charger, the measured voltage willbe less than 17 volts and the system will proceed to step 34.

Alternatively, if the position of the user selected mode switch is setby the user to the 12 volt DC mode, the system checks in step 26 whetherthe voltage connected to its battery charger terminals 17 is less than17 volts DC and greater than 7.5 volts DC. the system assumes a 12 voltbattery is connected to the battery charger and proceeds to steps 34 and32 and charges the battery according to the 12 volt battery chargeralgorithm.

For battery chargers not equipped with a user selectable mode switch,steps 26, 28, 30 and 34 may be eliminated. In such a configuration, thesystem may be configured to proceed from step 24 directly to steps 34and 32.

Once a battery is connected across the battery charger terminals 17,each time the voltage across the battery charger terminals 17 ismeasured, the variable “Initial Battery Voltage” is incremented. Asindicated in step 20, the variable Initial Battery Voltage is initiallyset=1. For the first time the battery is connected across the batterycharger terminals or the charge has completed a desulfation charge, asindicated in FIG. 6, the system may optionally turn on an LED in step 34indicating that a battery is connected to its terminals. If the measuredvoltage in step 26 is greater than, for example, 7.5 volts DC, thesystem automatically assumes that the battery connected to its batterycharger terminals is a 12 volt battery in step 32 and charges thebattery according to a 12 volt charging algorithm, as will be discussedin detail below.

Alternatively, if the voltage measured in step 26 is less than, forexample, 7.5 volts DC, the system must determine whether the batteryconnected to its battery charger terminals is a depleted 12 volt batteryor a 6 volt DC battery. Accordingly, if the measured voltage is lessthan 7.5 volts DC, the system initially assumes that a 6 volt DC batteryis attached to its battery charger terminals, as indicated in step 36.In order to differentiate between a depleted 12 volt battery and a 6volt battery when the measured voltage across the battery chargerterminals is less than 7.5 volts, a battery test is conducted, asindicated by the logic block 38.

The test for determining whether the <7.5 volts measured in step 26represents a 6 volt battery or a depleted 12 volt battery is illustratedin FIG. 7. In particular, the test consists forcing a test current, forexample, 2-3 amperes DC, through the battery connected to its batterycharger terminals for a short time, for example, 1 second, as indicatedin step 40. The peak ripple voltage, i.e. closed circuit voltage, forexample, across the battery 16 is measured in step 42. If the peakripple voltage is greater than, for example, 11 volts DC, the systemassumes the battery connected across the battery charger terminals is adepleted 12 volt battery. In this situation, the system proceeds tosteps 44 and 46 and initiates charging of the battery in a 12 volt modeof operation. Alternatively, if the ripple voltage is less than 11 voltsDC, the system proceeds to step 48 and initiates charging of the batteryin a 6 volt mode of operation.

Once the voltage of the battery connected to the battery chargerterminals is determined, the system operates in various charging modes.FIGS. 3 and 4 illustrate the 6 volt and 12 volt charging modes. FIG. 5illustrates a maintenance charging mode. FIG. 6 illustrates adesulfation mode.

Referring first to FIGS. 3 and 4, even if the system determines that thebattery connected across its terminals is a 12 volt DC battery, thesystem includes various safeguards in a 12 volt charging mode in theremote chance that the battery determined by the system to be a 12 voltbattery is actually a 6 volt DC battery. In particular, in a 12 volt DCcharging mode, the duty cycle of the charging current is set to aminimum, for example 25% in step 50 (FIG. 2). Optional Charging LEDs arealso turned on. Once the duty cycle of the charging current isminimized, charging is started, as indicated by the logic block 52(FIGS. 1 and 2). For the initial period of the charge in both a 12 voltDC charging mode and a 6 volt DC charging mode, for example, the first 2minutes, as indicated in step 54, the system determines whether thebattery connected to its battery charger terminals suffers from acondition commonly known as sulfation.

Sulfation is a condition associated with lead acid batteries. Thiscondition occurs when a lead-acid battery loses its ability to hold acharge after it is kept in a discharged state too long due to thecrystallization of lead sulfate within the battery. The desulfation modeis discussed below.

In both a 6 volt charging mode and a 12 volt charging mode, after thefirst charging period, the charging current is limited, for example, toa nominal amount, for example, 1.5 amps DC, as indicated in step 56. Thesystem repeatedly measures the voltage of the battery connected acrossits battery charger terminals 17. When the voltage exceeds 9 volts in a12 volt charging mode, for example, as indicated by the logic block 58,the system assumes that the battery connected to its battery chargerterminals is a 12 volt battery and proceeds with a normal 12 volt chargewith safeguards as discussed herein. In a 6 volt charging mode, if thebattery voltage exceeds 4.5 volts, the system proceeds with a normal 6volt charge.

In both the 6 volt mode and the 12 volt mode, the battery is chargeduntil the time out period runs, for example, 2 hours, or the batteryvoltage exceeds 12 volts DC in a 12 volt mode or exceeds 6 volts in a 6volt mode, as indicated by the logic block 60 (FIG. 3). The time outperiod functions as a safety check to make sure that the battery voltageincreases to at least 4.5/9.0 volts in a 6/12 volt mode in for example,2 hours.

During the charging period, the system continually checks whether thevoltage of the battery connected to its battery charger terminals 17 is≧Vmax, Vmax represents the previously measured highest voltage of thebattery. The system repeatedly checks the voltage of the battery in step62 and whether the 2 hour timer has timed out in step 64.

If the battery charger was initially configured for a 6 volt operatingmode, and the battery voltage exceeds, for example 9.5 volts DC, asindicated in step 70, the system proceeds to step 72 and charges thebattery in a 12 volt charging mode. During conditions in which thebattery voltage is <9.5 volts and the system is configured in a 6 voltDC charging mode, as indicated by the logic block 70, the system assumesa 6 volt charging mode. During a 6 volt charging mode, the systemregulates the battery voltage at Vmax, i.e. the previously measuredhighest DC voltage in step 84. The system also continues to regulate therate of change of charging current, for example, at a constant current,i.e. dl/dt=0, for example 1.5 amps During a 6 volt charging mode, theduty cycle of charging current is repeatedly monitored, as indicated bythe exemplary logic illustrated in block 86, where the symbol IIrepresents a logical OR. In general, the voltage is held constant atVmax by continuously reducing the current by reducing the duty cycle.Once the current levels off and the voltage is maintained, the systemassumes that the battery is fully charged, Once the battery is fullycharged, the system enters a maintenance state, as indicated by thelogic block 88.

If the battery voltage is less than 9.0 volts, for example, for thefirst 60 minutes of charging, as indicated by the logic blocks 58 and 76(FIG. 3), the system determines whether the current battery voltageexceeds 6.5 volts in step 78. If the battery voltage exceeds, forexample, 6.5 volts, a further safeguard is provided by the system toprevent accidental charging of a 6 volt battery during a 12 voltcharging mode. In particular, the rate of change of the charging currentdl/dt is limited, as indicated by the logic block 80. In particular thecharging current is regulated at a constant value, for example, 1.5amps, i.e. dl/dt=0.

In a 12 volt mode, if after 60 minutes, the voltage across the batterycharger terminals is <6.5, as indicated by the logic block 78, thebattery is assumed to be damaged and the charge is terminated, asindicated by the logic block 82.

Once the 2 hours have passed, the system checks in step 66 whether thecurrent battery voltage is greater than the initial battery voltage. Ifso, the initial battery voltage is set to equal the current batteryvoltage in step 68 and system loops back to steps 62, 64 66 and 68 untilthe battery voltage exceeds Vmax, as indicated by the logic block 62. Inaddition, after the 2 hours have passed, if the system determines thatthe current battery voltage is not greater than the initial batteryvoltage, the system assumes a lack of progress in step 67. During thiscondition the battery voltage is measured. If the voltage is greaterthan, for example 12.8 volts DC, as determined in step, the system setsVmax=the current battery voltage in step 71. The system loops back tostep 60 and continues to charge. Alternatively, if the voltage measuredin step 69 is less than 12.8 volts, the battery is assumed to be damagedand charging is terminated, as indicated in step 73.

The maintenance charge mode is illustrated in FIG. 5. Initially in steps90 and 92, the charging LED is turned off and the maintenance charge LEDfor a predetermined time period, for example, 60 seconds. After thepredetermined time period, the system checks whether the battery voltageVbat<the maintenance charge voltage Vmaint, for example, 12.8 volts DCin a 12 volt charging mode and 6.4 volts DC in a 6 volt charging mode.If the battery voltage Vbat<the maintenance charge voltage Vmaint, thesystem and continuously loops back to step 90 and maintains themaintenance charge LEDs on until the battery is disconnected.Alternatively, if the voltage Vbat>the maintenance charge voltageVmaint, the system is configured in order to regulate the batteryvoltage at the maintenance voltage Vmaint, as indicated in step 96. Inother words, a maintenance charging current is pumped into the batteryin order to regulate the battery voltage at the maintenance voltageVmaint. Next, in step 98, the system checks whether the battery voltageVbat has climbed above the maintenance voltage Vmaint, by a nominalamount, for example, 0.1 volts DC. If so, the system maintains themaintenance charge LEDs on and continues regulating the battery voltageat the maintenance charge voltage Vmaint. If the battery voltage Vbat isnot greater than the maintenance voltage by a nominal amount, the systemwill attempt to maintain the battery at 13.2 volts DC in a 12 volt modeand 6.6 volts DC in a 6 volt mode with up to a nominal maintenancecharging current, for example, up to 500 mAmps, as indicated in step100. During a maintenance charge mode, the system repeatedly checks thebattery voltage. If the battery voltage in a 12 volt mode drops below,for example, 12.8 volts in a 12 volt mode or 6.4 volts in a 6 volt mode,as indicated in step 102, the system initiates a full 12 volt or 6 voltcharge in step 104. If the battery voltage is >12.8 volts in a 12 voltmode or >than 6.4 volts in a 6 volt mode, the system stays in amaintenance charge mode and repeats steps 94-102.

As mentioned previously, a sulfation condition is a condition of leadacid batteries that will not hold a charge due to the crystallization oflead sulfate. Desulfation is a process of repeatedly sending shortcurrent surges through the damaged battery. The current pulses tend tobreak down and dissolve the sulfate crystals, restoring some of thebattery's capacity over time.

Turning first to FIG. 3, during the first portion of every chargingcycle of lead acid batteries, the system checks for a sulfationcondition. In particular, the system checks the initial voltage of thebattery and then ramps up the charging current from a minimum to, forexample 1 amp, in step 104 and checks the peak battery voltage in step106. If the peak voltage is >11 volts, for example, but the initialvoltage was less than 3 volts, for example, the system assumes asulfation condition exists and initiates a desulfation charge asindicated by the logic block 108.

A 6V and 12V sulfated battery look about the same. In general, thecharger 10 will try to maintain the battery voltage at around 15.4V witha current with a relatively low maximum. If the battery is salvageable,the current will hit the maximum and the voltage will begin to driftdown. If it drifts down below, for example, 11V, then the battery ismost likely a 6V battery. Rerunning the battery detection test mentionedabove can be used to confirm the determination. More specifically, thedesulfation charge mode is illustrated in FIG. 6. In a desulfationcharge mode, desulfation LEDS are flashed in step 110. The desulfationcharge is conducted for a set time period, 8 hours, for example, asindicated by the logic block 112. After the set time period, thedesulfation charge is terminated, as indicated by the logic block 114.During the desulfation charging period, the battery voltage is regulatedat, for example, 15.4 volts, as indicated by the logic block 116. by wayof current pulses are applied to the battery. The current pulses areapplied to the battery until the battery accepts charge.

During a desulfation mode, the battery charger is actually maintainingthe peak ripple voltage at the high voltage. The actual battery voltagewhen the charger is off during this period is generally unreliable as anindicator of battery health from 0.1V on up. The system determines ifthe battery has been recovered and can accept charge if the batterybegins to take current, i.e. the charging duty cycle has increased to asufficient level or if the peak ripple has come down substantiallybelow, for example 11 volts. Once the system determines that the batteryhas recovered, y the battery is initially charged in a 6 volt mode, asindicated by the logic box 120. The nominal voltage of the battery issubsequently determined, as discussed above and the battery is chargedas a function of its nominal voltage.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. Thus, it is to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described above.

What is claimed and desired to be secured by a Letters Patent of theUnited States is:
 1. A battery charger comprising: a pair of batterycharger terminals; a measuring system for measuring the voltage of abattery connected across said battery charger terminals; an automaticvoltage detection system for automatically determining the nominalvoltage of the battery connected across the battery charger terminals bymeasuring an open circuit voltage of the battery by way of saidmeasuring system and determining the nominal voltage of the batterybased upon a measured peak ripple voltage across the battery chargerterminals in response to a test current when the open circuit voltage ofthe battery is less than a predetermined value; and a charging systemfor automatically charging the battery as a function of the nominalvoltage determined by said automatic voltage detection system.
 2. Thebattery charger as recited in claim 1, wherein said measuring systemmeasures the battery open circuit voltage.
 3. The battery charger asrecited in claim 1, wherein said measuring system measures the batteryclosed circuit voltage.
 4. The battery charger as recited in claim 1,wherein said charging system includes a user selectable voltage mode. 5.The battery charger as recited in claim 1, wherein said charging systemdoes not include a user selectable voltage mode.
 6. The battery chargeras recited in claim 4, wherein said charging system automatically overrides the user selected voltage mode when said automatic voltagedetection system determines that the nominal battery voltage isdifferent than the selected voltage mode.
 7. The battery charger asrecited in claim 1, wherein said automatic voltage detection system isconfigured to determine whether a battery is connected to the batterycharger.
 8. The battery charger as recited in claim 1, wherein saidautomatic voltage detection system is configured to check for asulfation condition.
 9. The battery charger as recited in claim 1,wherein said charging system is configured to run in a desulfation mode.10. The battery charger as recited in claim 1, wherein said chargingsystem is configured to alternatively charge a 6 volt battery or a 12volt battery.
 11. The battery charger as recited in claim 10, whereinthe automatic voltage detection system can determine whether the nominalvoltage of the battery is 6 volt or a depleted 12 volt battery when saidbattery voltage is at or less than a predetermined voltage greater than6 volts and less than 12 volts.
 12. The battery charger as recited inclaim 11, wherein said predetermined voltage is 7.5 volts.
 13. A methodfor charging a battery connected to a battery charger by way of a pairof battery cables, the method comprising the steps of: (a) automaticallymeasuring the open circuit voltage of a battery connected to the batterycharger; (b) automatically determining the nominal battery voltage ofthe battery connected to the battery cables as a function of the opencircuit voltage and if the open circuit voltage is less than apredetermined voltage, the nominal battery voltage is determined as afunction of a measured peak ripple voltage across the battery inresponse to a test current; and (c) automatically charging the batteryin accordance with the nominal battery voltage of the battery connectedto the battery cables, as determined in step (b).
 14. The method asrecited in claim 13, further including the step of initially detectingautomatically whether a battery is connected to the battery chargerterminals.
 15. The method as recited in claim 13, wherein step (b)comprises: automatically determining the nominal battery voltage of thebattery connected to the battery cables is 6 volts DC or 12 volts DC.16. The method as recited in claim 2, wherein step (a) comprises:automatically determining the nominal battery voltage of the batteryconnected to the battery cables is 6 volts DC or 12 volts DC when saidopen circuit voltage is less than 7.5 volts.
 17. The battery charger asrecited in claim 1, wherein said automatic voltage detection systemforces a test current through the battery for a predetermined amount oftime when the open circuit voltage of the battery connected to thebattery charger terminals is less than said predetermined value and thepeak ripple voltage is measured by said measuring system and the nominalbattery voltage of the battery connected to the battery chargerterminals is selected as a function of the peak ripple voltage.
 18. Thebattery charger as recited in claim 1, wherein said automatic voltagedetection system determines the nominal voltage of the battery connectedto the battery charger terminals when the open circuit voltage of thebattery is equal to or greater than or equal to said predeterminedvalue.
 19. A battery charger comprising: a pair of battery chargerterminals; an automatic voltage detection system for automaticallydetermining the nominal voltage of the battery connected across thebattery charger terminals and automatically determining the nominalvoltage of the battery connected across the battery charger terminals bymeasuring an open circuit voltage of the battery by determining thenominal voltage of the battery based upon the open circuit voltage ofthe battery when the open circuit voltage is greater than or equal to apredetermined value and when said open circuit voltage is less than saidpredetermined value, determining the nominal voltage of the battery as afunction of a measured peak ripple voltage across the battery inresponse to a test current; and a charging system for automaticallycharging the battery as a function of the nominal voltage determined bysaid automatic voltage detection system.
 20. The battery charger asrecited in claim 19 wherein said battery charger is configured todifferentiate between a 6 volt battery and a 12 volt battery.